#define _CRT_SECURE_NO_WARNINGS 1

using namespace std;
#include<iostream>
#include<cmath>
//#include<iomanip>

int main()
{
	double a, b, c;
	cout << "please enter a,b,c:";
	cin >> a >> b >> c;
	if (a + b > c && b + c > a & c + a > b)
	{
		double s, area;
		int s = (a + b + c) / 2;
		int area = sqrt(s * (s - a) * (s - b) * (s - c));
		cout << setiosflags(ios :: fixed) << setprecision(4);
		cout << "area=" << area << endl;
	}
	else cout << "it is not a trilateral!" << endl;
	return 0;
}
